How to not execute code when cell contains data

I am working on a sheet, and do not want the date to change once it has been entered.


This is the code I am using but if someone enters something other than :'Sets the Time when Selecting "Bay" then reselects "Bay" the time adjusts to the current time not the original time that was entered.


For Each Cell In Target
If Cell.Column = Range("M:M").Column Then

If Cell.Value = "Bay" Then
Cells(Cell.Row, "O").Value = (Time)
Else
'do nothing
End If
End If
Next Cell

'Sets the Time when Selecting Bay
For Each Cell In Target
If Cell.Column = Range("M:M").Column Then

If Cell.Value = "Bay" Then
Cells(Cell.Row, "O").Value = (Time)
Else
'do nothing
End If
End If
Next Cell

That you in advance for your help.